1. Field of the Invention
The invention relates to the formation of sound insulating linings for gas ducts such as, for example, the secondary flow paths of bypass jet engines for aircraft.
Jet engines give rise to numerous sources of noise during their operation and FIG. 1 shows, relative to the axis of a jet engine, the directional pattern of the various sound waves which are generated.
A first noise source is the internal noise which is generated by the combustion and the expansion of hot gases through the turbine, and which is emitted as a beam 1 directed downstream of the engine.
A second noise source is the jet noise 2, which is also directed downstream of the engine and is generated in the zone where the gas flows mix and where turbulence is very considerable by virtue of the substantial differences in velocity between the flows (i.e. hot flow and cold flow, cold flow and ambient air).
Another noise source is that caused by the medium pressure compressor, and in this case the noise 3 is directed upstream of the engine.
Finally, a further source of noise is created by the fan disposed in the bypass flow. The fan noise propagates as two lobes 4 and 5, one directed in front of the engine, and the other directed downstream. Although at a level slightly below the jet noise, the fan noise has the drawback that its propagation lobes are very widely spread laterally relative to the aircraft.
2. Description of the prior art.
The imposition of regulations to reduce noise levels in the vicinity of airports has led to a very close interest being taken in the means for reducing the fan generated noise. One of these means consists of using on the inner part of the outer cold flow casing coatings of porous materials of good absorbing quality or sound insulating linings with Helmholtz resonators.
In the latter case it has already been proposed in U.S. Patent No. 4,449,607 to construct a sound insulating lining on the wall of a gas flow path, the lining comprising a plurality of thin perforated panels which overlap end to end like tiles and, between the wall and each of the perforated panels, an intermediate layer designed to define with the perforated panel a plurality of resonant acoustic cavities having acoustic vents formed by the holes of the perforated panel, each panel being fixed directly to the wall by screws located through struts, and holding the intermediate layer between itself and the wall.
Such a lining, though very efficient, suffers nevertheless from the drawback of being relatively complicated to make and to assemble, as requires making complicated struts and washers, which must be adhesively bonded to opposite sides of the perforated panels.